For various applications, it is beneficial to not merely guide light through a waveguide, but to also interact with the guided light, thereby achieving e.g. optical detection, modulation and/or attenuation. In such applications, electro-optic devices are used. Examples of such electro-optic devices are photodetectors, modulators, lasers, and variable optical attenuators. A known way to accomplish modulation is, for example, through the Franz-Keldysh effect or the quantum-confined Stark effect.
Basic components used in electro-optic devices for inducing an electric field are diodes, built around a portion of the waveguide. Two types of p-i-n diode waveguides that can be used are lateral p-i-n (LPIN) and vertical p-i-n (VPIN) diode rib waveguides. In a LPIN rib waveguide, the p-i-n diode is arranged in a lateral configuration, whereas in a VPIN diode rib waveguide, the p-i-n diode is arranged as a vertically stacked combination of p-type, intrinsic and n-type layers. VPIN diode waveguides are able to provide higher electric fields overlapping with a larger area of the guided light, especially for waveguides that have smaller waveguide thickness than waveguide width. An example of a cross-sectional view of a VPIN diode rib waveguide is shown in FIG. 1, whereby a p-type layer 10, an intrinsic layer 11, and an n-type layer 12 is configured in an optical stack, forming the waveguide wherein the optical mode 13 is propagating. Furthermore, contacting elements 14 and 15, according to some conventional examples, are shown to form the cathode and the anode.
Some VPIN diode waveguides may suffer from high metal induced absorption. Metallic contacts in the vicinity of the waveguide may, for example, overlap with and absorb an evanescent portion of the guided light. This absorption, particularly when they occur over an extended length, lead to significant undesired losses in the intensity of the guided light. Ways to alleviate this problem exist, but, to date, remain unsatisfactory. One example of a way to avoid absorption in VPIN diode waveguides is by providing contacts away from the PIN stack region, but this typically requires deposition of additional layers resulting in more processing complexity, and larger diode contact resistance. Another example is the use of designs in the VPIN diode waveguide whereby a whispering gallery mode is used for pushing away the light from the metal contact. Nevertheless, for high refractive index materials, long linear waveguide devices such as electro-absorption modulators and photodetectors cannot be realized with this approach.
Thus, there is still a need in the art for VPIN diodes comprising a waveguide portion that address some or all of the above-mentioned issues.